The issues of authentication and counterfeit deterrence can be important in many contexts. Bills of currency, stock and bond certificates, credit cards, passports, drivers licenses, as well as many other legal documents (e.g., deeds, wills, etc.) all must be reliably authentic to be useful. Museums and art galleries face such challenges when authenticating works of art. Additionally, consumer products and other articles of manufacturing, such as pharmaceuticals, books, movies, software, etc., are frequently the subject of counterfeiting in the form of “pirated” versions or “knock-offs.”
A wide variety of attempts have been made to limit the likelihood of counterfeiting. Most such attempts tend to incorporate a unique identifier into the potentially counterfeited item. The addition of fluorescent compounds to inks and dyes has long been a technique used by governments and banks for anti-counterfeiting purposes. Likewise, fluorescent compounds can be incorporated or otherwise associated with other articles for identification and/or anti-piracy purposes. See, e.g., U.S. Pat. Nos. 4,558,224 and 6,246,061.
Fluorescence, being a subset of photoluminescence (PL), occurs when a material is irradiated with electromagnetic radiation (EM), at least some of which is absorbed. Fluorescence refers to the subsequently re-emitted radiation of wavelength other than that which was absorbed. Typically, such emission, or fluorescence, is red-shifted to longer wavelengths relative to the incident or absorbed radiation, such emission can also be described as being Stokes shifted. The terms “fluorescence,” “luminescence,” and “photoluminescence,” will be used synonymously herein.
Fluorescent compounds typically used in such above-described applications are generally organic molecules that fluoresce in the visible region of the EM spectrum when irradiated with ultra-violet (UV) light. There is, however, a constant need for both new and better fluorescent compounds to a) stay ahead of the would-be counterfeiters, and b) to expand the breadth of such marking and authentication techniques, wherein such fluorescent compounds offer a more unique optical signal and/or yield themselves to processing and operating conditions unsuitable for existing fluorescent compounds.